Quantcast Functional Units of a Computer System

electrical, electronic, and magnetic devices within the computer itself and all related PERIPHERAL devices. The peripheral devices consist of items, such as a keyboard, magnetic tape unit, mouse, scanner, printer, and  so  on.  The  software  items  are  programs  and operating aids written so the computer can process data. The manufacturer supplies much of the initial software for a particular computer. This software is known as SYSTEMS   SOFTWARE.   Systems   software   is designed for broad general use. Examples of systems software are DOS (disk operating system) for IBM compatible  computers  and  System  7  for  Apple computers.  Software  designed  to  meet  a  specific requirement  or  purpose  is  called  APPLICATION SOFTWARE.  Examples  of  application  software  are WordPerfect,  Lotus  1-2-3,  and  Adobe  Photoshop. FUNCTIONAL UNITS OF A COMPUTER SYSTEM Digital computer systems consist of three distinct units. These units are as follows: Input  unit Central  Processing  unit Output  unit These units are interconnected by electrical cables to permit communication between them. This allows the computer to function as a system. Input  Unit A computer must receive both data and program statements to function properly and be able to solve problems. The method of feeding data and programs to a  computer  is  accomplished  by  an  input  device. Computer input devices read data from a source, such as magnetic disks, and translate that data into electronic impulses for transfer into the CPU. Some typical input devices are a keyboard, a mouse, or a scanner. Central  Processing  Unit The brain of a computer system is the central processing  unit  (CPU).  The  CPU  processes  data transferred to it from one of the various input devices. It then transfers either an intermediate or final result of the CPU to one or more output devices. A central control section and work areas are required to perform calculations  or  manipulate  data.  The  CPU  is  the computing center of the system. It consists of a control section, an arithmetic-logic section (fig. 3-1), and an internal storage section (main memory). Each section within the CPU serves a specific function and has a particular relationship with the other sections within the CPU. CONTROL  SECTION.—The  control  section directs the flow of traffic (operations) and data. It also maintains order within the computer. The flow of control is indicated by dotted arrows in figure 3-1. The control section selects one program statement at a time from  the  program  storage  area,  interprets  the  statement, and sends the appropriate electronic impulses to the arithmetic-logic and storage sections so they can carry out the instructions. The control section does not perform actual processing operations on the data. The control section instructs the input device on when to start and stop transferring data to the input storage area. It also tells the output device when to start and stop receiving data from the output storage area. ARITHMETIC-LOGIC   SECTION.—The arithmetic-logic   section   performs   arithmetic operations,   such   as   addition,   subtraction, multiplication,  and  division.  Through  internal  logic capability, it tests various conditions encountered during processing and takes action based on the result. As indicated by the solid arrows in figure 3-1, data flows between  the  arithmetic-logic  section  and  the  internal storage section during processing. Specifically, data is transferred as needed from the storage section to the arithmetic-logic  section,  processed,  and  returned  to internal storage. At no time does processing take place in  the  storage  section.  Data  maybe  transferred  back  and forth between these two sections several times before processing   is   completed.   The   results   are   then transferred from internal storage to an output device, as indicated by the solid arrow in figure 3-1. INTERNAL  STORAGE  SECTION.—The  internal storage section is sometimes called primary storage, main storage, or main memory, because this section functions similar to our own human memory. The  storage  section  serves  four  purposes;  three relate to retention (holding) of data during processing. First, as indicated by the solid arrow (fig. 3-1), data is transferred  from  an  input  device  to  the  INPUT STORAGE AREA where it remains until the computer is  ready  to  process  it.  Second,  a  WORKING STORAGE AREA ("scratch pad" memory) within the storage section holds both the data being processed and the   intermediate   results   of   the   arithmetic-logic operations.  Third,  the  storage  section  retains  the 3-2


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